In real races, to my knowledge, they do not refuel. Thus, you must set your fuel so you have enough, but not too much so you can be competitive, and there is the joy, working out your levels and speed. Seeing as practice sessions and quali and race times vary, it makes for interesting racing. Set your A.I level and go from there. They also do not stop for tires, just race from start to finish. So also, be careful of your tyre wear.
Hope that helped.
However, you can make them stop by using the PitStop Strategies in each track .gdb file. Add the lines at the end of the file. Use Notepadd + + to edit.